This invention relates to borescopes or endoscopes, and is particularly concerned with construction of the tube body of the borescope or endoscope insertion tube. The invention is more particularly concerned with construction of the tube body with terminators at its proximal and distal ends which provides a continuous electrical ground path between the end terminators, but minimizes problems of leakage current through the walls of the tube body.
A borescope is generally characterized as an elongated flexible insertion tube with a viewing head at its distal or forward end, and a control section at its proximal end for controlling the bending at the distal end. In such a borescope, a bendable tube steering section is situated at the distal end adjacent to the viewing head. Typically, control cables extend through the bendable tube section and the remainder of the insertion tube and connect with a steering control mechanism in the control section. These cables are differentially displaced for bending the steering section to facilitate the inspection of a remote object.
A borescope is typically intended for visual inspection of an intricate mechanical assembly, such as a jet engine or turbine, or a heat exchanger tube, where it would be difficult or impossible otherwise to view the assembly's internal elements. The borescope needs to be insertable into narrow, tortuous passageways, and must observe very delicate steering considerations.
An endoscope is typically inserted into a body cavity of a human or veterinary patient for visual inspection of tissue within the cavity. Because body passages such as esophagus, bronchi, and colon are narrow and tortuous, the steering section must be bent rather precisely, and as close to the viewing head as possible.
Insertion tubes are designed to be torsionally stiff, and dimensionally stable both radially and axially. On the other hand, the insertion tube must be free to bend so as to follow contours of the machine passages or body cavities where it is employed.
In a number of applications of either a borescope or endoscope, ac leakage current through the insertion tube wall can affect a process or can endanger the health of a patient. This leakage current comes through capacitive effects with respect to the surroundings. On the other hand, a good ground path needs to be provided through the insertion tube body for providing an electrical ground to the miniature video camera and/or other elements situated at the distal tip of the insertion tube.
A typical insertion tube is constructed on an inner steel coil which provides radial stability (i.e., crush resistance) but still permits lateral flexibility for bending. A tubular layer of an elastomeric material, e.g. urethane, is disposed over this to seal the wall. Then tubular braid layer is positioned over the urethane tube to provide axial stability (i.e., stretch resistance) and torsional stability (twist resistance) and the tube is coated with a flexible insulating material, e.g. polyurethane.
In the past no effort was made to isolate the coil and braid electrically at the ends where a terminator element is attached to couple to a steering control or to the distal steering element. As a result, the only dielectric or insulating layer between the interior of the insertion tube and its environment was the thin layer or coating over the braid. This permitted levels of leakage current that were unacceptable for many procedures.